Liquid level gauge



Oct. 1954 L. M.IOBERLIN 2,691,223

LIQUID LEVEL GAUGE Filed Aug. 5, 1952 2 Shee ts-Sheet 1 IN V EN TOR.

Oct. 12, 1954 M. OBERLIN LIQUID LEVEL GAUGE 2 Sheets-Sheet 2 Filed Aug. 5, 1952 ii iii 5:35.61

w m m m BY Luabwm ATTORNEYS Patented Oct. 12, 1954 EIQUID LEVELGA'UGE wlayman .M. :Oberlin,. Dewey;'.;.0kla.-,i: assignor :to Phillips. Petroleum: Compamw a .corporation.01

; Delaware Application August 5, 1952; Srial Nd} 302F718- 10 Claims.

This inventionzirelateszto apparatusffor measuring liquid levels. :Inranother :aspectY ittrelates :to. apparatus in which the; :liquid: level: iSndGtBIT- mined by variation, in capacitance: between. a pair of. spaced condenserxelements.

This application'is a:'icontinuationeinepart of application SeriaLNo. 98.348, filed June 310',-..19.49, now Patent N o.-.2,622,442;

In recent years the marketingiofzthigh; vapor pressure hydrocarbons, .such :as propane :andrbutane, has :become a very :active' phaseiof'theixpetroleum; industry. These. fuels I are. transported inliquidaform, :under .highrpressures; by means of specially constructed:railroaditank.cars.

,;In order to determine," .whemnee'essary; the amount of'liquid hydrocarbon- .product; contained in. a. tank car, so-called if liquidi levelxslipstube .gauges have been widely used. 'rI heserdevices consist 20f. a hollow .tube; slidablwa'enterin'g. :the

-shellof. the tanlccar througha' suitable .zpacking gland in: thezdome offthe. car; i The? tubeiszfitted at its lower: end with a: checknvalvelzhaving': a small orifice, and at its upper: end v Withjfi} shutoif valve. The outside ofrthe1tubet is :markedrin inches: and fractions: of inches; and: a' suitable 'reference pointer iseattached tozthei shell of; the oar, or toits: dome. When theihmertendxoff: the tube is immersed in :the. liquidsand the ishutrofi valve :is opened. a. fineispray issues afromxathe vented-end thereof; whereas whenzthennnerfiend of thextubeisldisposedinzthejgas aboveithe liquid a. distinctive vapor-1 issues :from the" vented vvend of. the tube. Thus by:manipulatingmhatube anopera-tor can determinestheszliquidtlevel within the tankiby noting.thezslip'a'tub positionnat which. a change. in; the. character rcflthe:issued vapor takes place.

Aserious: disadvantage -to the .use 1of.:.these: slip .tube gauges, howeven: residesain the .factrthat the material being measurediandits vapor escape into the-atmosphere whenever. "a; liquid level: determination is made-thereby .crea-ting aedangerous explosionv hazard. In order :to overcome .this..diffioulty thereis. provided. in. accordance with the present invention -an. improved; type. of .gaugewhereinthe liquid level. is determined by a measurementofthe .capicitance between spaced ,platesdisposed .vvithinthe. liquid being measured. thus affording. a liquid.levelrdetermina- .tion without the escape .ofNa-ny of .the:meas.- .uredi liquid .or its. vapor.

It is an object of the. present invention: topro.- vide apparatus .for measuring aliquidlevelwhich is free frm..the'- disadvantages encounteredin vprior systems.

It is afurthen object to provide apparatus-in .12. which the liquid: whose level: iszbeing measured "constitutes a ":condenser 1 dielectricwhereby changes :"in ii liquid {inlet/e1. ducer respective 2changes:zinrcapacitance between a-"set: ofrspaee'd 5 condenser; elements:

I" It'is a further: object" to -prov ide :.a::1iquid-i1eve1 indicator which is' -simpleiin.- construction, reliable -in'ioperation, 1 rugged and durable;

. Various #other': objects. advantages. 'andisfeam i tures"ofi'this'invention shouldi'be'eome' apparent' to those skilled in thexartli'fromti. the following detaile'dsrdeseription takem inconjuhctiorr .with the accompanying; .drawin'gsrin which:v

Figure. 1:. is: a. fragmentary verticalzl'sectioniof azhi-ghpressure: tankxcarrin:'vvhichtcthis. inventiorr-randzother necessary valves. J and accessories appear ini outline;

' Figure? isxan: enlargedi detailediviewi oft-the .xupperr portion :iofizthe: liquid level. gauge .of:" this .invention;

Figure 31. is\ a: .zverticalzxsectional. vi'ew of the "lower; portionio'fwthe (liquid level gauge in thisfin- "vention;

Figure Atisxarvertical :sectional view-10f a modi- .ficationtofrlthe'invention;

Figure 51 is; avertical sectional: :view of.v a sec- =ondumodificationcof:the: invention; .and-

3 Figurerdvis a schematicwviewnofrthei electrical circuitryremployed'in con-junctionwith the: liquid levelugauge.

' Referringsncwrtmtheadrawingsin2detail and-rte "Figures 1:;andz2 in: partidulargthenumeral. 1| 0 designates the central :lSBCfiDIl ofifthe'tankzshell 'whicl ris filleiwithili uified petrcleumigasctoxithe level- :den'otedvby the: surfaced I Argaugei tube LI 2 .55 islidablyxenters the; top'rof: the: tank 1; through: a

lantern. gland ll 3 candii itszupperrend extendsr-into 1 thergauge housing I 4-: A casing I iirenclosin'g' two npair of' spaced :capacitorxplates: is'.;attached ito *thedowerxendrof .tube 12. Tubewm-"normallyiis maintained: in; the position illustratedbyxmeans of, ahold-down: arm =16 which is amounte'dz'with in: :housing 4 sons: tohe capable .ofrotationuin a. horizontals plane.- Theupper'endeof tube ilZiis provided: with: a flange member 11- whichznonvmally is zin-.eng agement with the lower' -edge'sof ,holdedown arm-#16: P A: horizontal -red'ge:'of-'anm -l 6 canzserve as? a reference-pointer in measuring theliquid level with-reference a to they graduations appearing :on .the upper"portion of:.tubev I 2.

In transit arm 1 61-. isolocatedbyv suitable means whichrpcsition. it over. flange I l thus aretaining gauge tube I 2. substantially within... the interior .ofrthewtank. Whenever a determinationisofsthe .liquiddevel intheHtank. is made chain 'l9ris-i'unzfastened and-gauge housing Ill-removed: Hold;-

down arm |6 then is swung aside allowing gauge tube l2 to be raised out of the tank. In order to prevent tube l2 from being expelled rapidly from the tank by the pressure therein, a restraining arm 2| disposed in approximately horizontal position is pivotally mounted on a stanchion 22 by means of a pin 23 disposed in a slot 24. This pivotal mounting permits angular movement of arm 2| in a vertical plane. A hole I8 is provided in arm 2| such that rod I2 is free to move therethrough whenever restraining arm 2| is in a horizontal position. A spring 26, however, normally urges rod 2| upward away from the horizontal position which results in the introduction of friction and binding between the gauge tube and the restraining arm. The extended end of arm 2| serves as a handle or grip. By lowering the extended end of arm 2| the friction between rod I2 and arm 2| is removed and gauge l2 can be raised or lowered with respect to the tank. Other valves necessary to the use, maintenance and protection of a typical tank car are shown within the dome housing, but are not described in detail because they form no part of the present invention and are described in U. S. Patents 1,827,574 and 2,361,872.

In Figure 3 casing which is attached to the lower end of rod |2 by screws such as 29 is illustrated in greater detail. This casing is formed by elongated support members 30 and 3| joined by transverse supports, one of which is shown at 32, the latter being provided with openings 33. Support members 36, 3| carry a first pair of elongated relatively narrow metal strips 35, 36, respectively, mounted in facing, closely spaced, parallel arrangement. A second pair of relatively narrow metal strips 37, 38 are mounted in facing, closely spaced, parallel arrangement on respective members 36, 3| below strips 35, 36. The two sets of strips preferably are the same length although this is'not essential for proper functioning of the apparatus. Electrical leads 4|], 4|, 42 and 43 connect respective plates 35, 36, 37 and 38 to the bridge circuit illustrated in Figure 6 which is disposed outside of the fiuid tank in a housing 44 mounted on plate I6. Leads 40, 4|, 42 and 43 pass through a passage 45 in casing l5 and are enclosed within a suitable flexible fluid-tight conduit 46 which is connected between casing l5 and the housing 44 disposed on plate l6. Conduit 46 passes through a small opening in plate |6 which is provided with a suitable sealing device 47. A plurality of openings 33 are provided in casing l5 to admit liquid into the interior of the casing such that the liquid level within casing I5 is always the same as the liquid level outside the casing. It should thus be apparent that metal strips 35, 36 form a first pair of spaced condenser elements and strips 37, 38 form a second pair of spaced condenser elements. If these spaced elements are immersed within the liquid, the liquid forms the dielectric between said elements; and if the elements are in the region of vapor above the liquid, this vapor forms the dielectric material between said elements. If the liquid in the tank is electrically conductive the strips can be coated with a thin layer of insulating material. In such case the term dielectric material as used herein still refers to the liquid between the layers of insulating material.

A suitable bridge circuit adapted to compare the capacitance between plates 35, 36 with the capacitance between plates 37, 38 is illustrated in Figure 6. The bridge components include an alternating current source 56, a switch 5|, a galvanometer 52 and variable balance resistors 53 and 54. The condenser formed by plates 35, 36 forms one arm of the Wheatstone bridge circuit and the condenser formed by plates 37, 38 forms a second arm of the bridge circuit; resistors 53 and 54 form the third and fourth arms of the bridge circuit.

In operation of the capacitive type gauge herein described, casing |5 first is raised and then slowly lowered into the liquid under measurement until the bridge circuit is balanced, that is, at which time the capacitance between plates 35, 36 is equal to the capacitance between plates 37, 38. This is indicated by a zero deflection of galvanometer 52 if resistors 53 and 54 have equal impedance. Before balance is obtained a portion of the space between plates 35, 36 is filled with the vapor above the liquid, which vapor has a dielectric constant different from the dielectric constant of the liquid. At the point where balance is obtained it is known that the liquid level H is at the top end of plates 35, 36 and the depth of level H is read on the graduated scale of rod l2. It should be noted that successful operation of this gauge does not depend in any way upon the dielectric constant of the liquid under measurement since the same liquid forms the dielectric of both condenser units.

As previously stated, the bridge circuit is balanced when the liquid level reaches the upper end of plates 35, 36. However, if the depth of the liquid is not sufficient to reach the top end of plates 35, 36 the bridge circuit can be adjusted for balance when a preselected relationship exists between the capacitance of the condenser units rather than when the capacitances are equal. This is accomplished by suitable calibration of galvanometer 52 or adjustable resistors 53, 54. It is, of course, impossible with the embodiment of Figure 3 to determine the depth of the liquid level if said liquid level is below plates 35, 36. This difi'iculty can be overcome, however, by employing the apparatus of Figure 4 in which two casing members and BI are connected by a hinge 63. Casing section 60 carries plates 35, 36 and casing section 6| carries plates 37, 38. The two casing sections preferably are biased by a spring 62 to a position wherein one section is perpendicular to the other which results in plates 37, 38 always being immersed in the liquid being measured. With this modification readings can be taken accurately even where the liquid level is quite low because the plates 37, 33 are positioned much closer to the bottom of the tank.

A still further modification of this invention which is adapted to accomplish the same purpose as the modification of Figure 4 is illustrated in Figure 5. In this embodiment a pair of interleaved multiple capacitor plate elements 65 and 66 are mounted immediately under plates 35, 36. Plates 65 and 66 correspond to respective plates 37 and 38 in Figure 3 except that by interleaving the several plates of 65, 66 a capacitance equal to the capacitance between plates 35, 36 is obtained in a much narrower space thereby permitting casing [5 to be lowered further in the tank.

From the foregoing description it should be apparent that there is provided in accordance with the present invention a liquid level indicator utilizing the dielectric property of the liquid whose level is to be measured as a means for determining its depth. The accuracy of the reading is not affected by the dielectric constant of the liquid and no calibration thereof is required when liquids of differing dielectric constants are stored at different times in a single tank. The gauge herein provided is particularly adapted for use in measuring the liquid level in tank cars carrying liquefied petroleum gases. In this respect readings can be obtained without venting the measured material into the atmosphere. It should be apparent, however, that while this invention has been described in conjunction with measuring the liquid level in tank cars, the principles described are applicable to measuring liquids in any type container.

While this invention has been described in connection with present preferred embodiments thereof, it is to be understood that this description is illustrative only and is not intended to limit th invention.

What is claimed is:

1. Apparatus for measuring a liquid level comprising, in combination, two units each including a pair of spaced condenser elements, the liquid whose level is to be measured constituting substantially the entire dielectric material for one of said units, means for moving the other unit into and out of said liquid, a scale for measuring the position of said other unit, and means for comparing the electrical capacitance of said units, said scale indicating the liquid level when a predetermined relation exists between the capacitance of said units.

2. Apparatus for measuring a liquid level comprising, in combination, a casing, two pairs of elongated relatively narrow metal plates mounted within said casing in closely spaced parallel arrangement, one pair of plates being mounted above the other pair of plates, said casing being perforated to allow flow of liquid into and out of the casing, means for raising and lowering said casing in a liquid Whose depth is to be ascertained, said means including a scale for indicating the position of the casing, and means for comparing the electrical capacitance of said units, said scale indicating the liquid level when a predetermined relation exists between the capacitance of said units.

3. Apparatus in accordance with claim 2 in which the spacing between and the relative areas of said two sets of plates are such that the capacitanccs between said two sets of plates are equal when materials having like dielectric constants are disposed between said two sets of plates.

4. Apparatus in accordance with claim 2 in which said two sets of plates are of equal length, said sets of plates have equal areas opposing one another, the individual plates of said two sets being equally spaced, and said scale indicating the liquid level when the capacitance of said pairs of plates is equal.

5. Apparatus in accordance with claim 4 in which the comparing means is an alternating current Wheatstone bridge circuit, said pairs of condenser plates forming two arms of the Wheatstone :bridge circuit.

6. Apparatus for measuring a liquid level, comprising, in combination, a hinged casing including an upper section and a lower section, a set of condenser elements mounted in the upper section, a set of condenser elements mounted in the lower section, each set including two elongated relatively narrow metal plates arranged in closely spaced parallel formation, means for immersing said casing in a liquid whose depth is to be measured, said means including a scale for indicating the position of the casing in the liquid, means for comparing the electrical capacitance of said sets of condenser elements, said scale indieating the liquid level when a predetermined relation exists between the capacitance of said sets of condenser elements, and means for urging the two sections of the casing to predetermined relative positions so that the upper section may be lowered substantially to the bottom of the tank while the lower section remains immersed in said liquid.

7. Apparatus in accordance with claim 6 in which the two sets of metal plates are of the same length, said scale indicating the liquid level when the capacitance of the condenser units is equal, and in which the balancing means comprises an alternating current Wheatstone bridge circuit, the two sets or" condenser elements forming two arms of the bridge circuit. I

8. Apparatus for measuring a liquid level comprising, in combination, a casing including an upper section and a lower section, a first set of condenser elements mounted in said upper section, said first set including two elongated relatively narrow metal plates arranged in closely spaced parallel formation, a second set of condenser elements mounted in said lower section, said second set including two interleaved metal plates arranged in closely spaced relationship whereby the capacitance of said first set of condenser elements is approximately equal to the capacitance of said second set of condenser elements when materials having like dielectric constants are disposed between said two sets of plates, means for immersing said casing in a liquid whose depth is to be measured, said means including a scale for indicating the position of the casing in the liquid, and means for comparing the electrical capacitance of said sets of condenser elements, said scale indicating the liquid level when a predetermined relation exists between the capacitance of said sets of condenser elements.

9. Apparatus in accordance with claim 8 in which the comparing means is an alternating current Wheatstone bridge circuit, said pairs of condenser plates forming two arms of the Wheatstone bridge circuit.

10. Apparatus for measuring the liquid level in a tank adapted to contain liquified petroleum gas comprising, in combination, a casing including an upper and a lower section, a set of capacitor elements mounted in each of said sections, said casing being perforated to allow flow of liquid into and out of said casing to form the dielectric material between said capacitor elements, a rod secured at one end to said casing to raise and lower said casing in said tank, a scale associated with said rod to indicate the position of said casing in said tank, an alternating current bridge circuit positioned outside said tank, and electrical leads connecting said sets of capacitor elements in two arms of said bridge circuit whereby said scale indicates the liquid level when a predetermined relation exists ibetween the capacitance of said sets of capacitor elements.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,695,701 Steiner Dec. 18, 1928 2,032,016 Hitner Feb. 25, 1936 2,357,023 Reid et a1 Aug. 29, 1944 2,480,490 Mark Aug. 30, 1949 2,511,398 De Giers June 13, 1950 2,541,743 Brockman Feb. 13, 1951 2,581,085 Edelman Jan. 1, 1952 

